1. Field of the Invention
This invention relates to a process for a potentially economically viable method for the preparation of reactive superoxide ion in deep eutectic solvents.
2. Background of the Related Art
Superoxide ion is a reactive oxygen species formed by the one electron reduction of oxygen, has a longer lifetime than singlet oxygen and is capable of decolorizing (bleaching) stains and killing bacteria. Throughout this application, superoxide is represented as O2•− based on common literature practice.
Superoxide is very reactive in aqueous solutions and protic solvents. On the other hand, O2•− is quite stable in aprotic solvents. In general, O2•− behaves as an oxidant, and as a strong nucleophile, depending on the solvent, in particular on the pH or presence of an easily abstractable hydrogen atom. Superoxide also acts as a one-electron reductant of metal ions and complexes.
O2•− has been known by chemists as long as 1934 when Haber and Weiss (J. Proc. R. Soc., 1934, A147, 332) have proposed that O2•− is formed in the decomposition of hydrogen peroxide and in the oxidation of ferrous ions by dioxygen in aqueous solutions. Sawyer and co-workers (Merritt, M. V. and Sawyer, D. T. J. Org. Chem. 1970, 35, 2157. Sugimoto, H.; Matsumoto, S.; and Sawyer, D. T. Environ. Sci. Technol., 1988, 22, 1182) pioneered work on superoxide ion, particularly the direct electrochemical reduction of dissolved oxygen gas in aprotic solvents to form O2•− according to the following reactionO2+e−→O2•−  (1)
A comprehensive review of superoxide ion chemistry is given by Sawyer et al. (Sawyer, D. T., Sobkowiaand, A. k, and Roberts, J. L. Electrochemistry for Chemists, 2nd ed., chapter 9, Wiley Interscience New York, 1995). Superoxide ion can be formed directly from solvation of KO2 in aprotic solvents, or electrochemically via direct cathodic reduction of dioxygen (typically E=−1.0V vs SCE). O2•− is a strong nucleophile and disproportionates in water to O2 and hydroperoxide:2O2•−+H2O→O2+HOO−+HO−  (2)
To avoid this reaction, generation and utilization of O2•− must be done in aprotic solvents. Acetonitrile (MeCN), dimethyl formamide (DMF) and dimethyl sulfoxide (DMSO) are commonly used.
Che et al. studied the water-induced disproportionation of the electrogenerated superoxide ion in MeCN, DMF, and DMSO media containing various concentrations of water using UV-vis spectroscopy (Che, Y.; Tsushima, M.; Matsumoto, F.; Okajima, T.; Tokuda, K.; and Ohsaka, T. J. Phys. Chem. 1996, 100, 20134).
In dipolar aprotic solvents superoxide ion is quite stable, because disproportionation to give the peroxide dianion (O22−) is highly unfavorable (Sawyer, D. T. Oxygen Chemistry; Oxford University Press: New York, 1991. Afanas'ev, I. B. Superoxide Ion. Chemistry and Biological Implications; CRC Press: Boca Raton, Fla., 1989; Vol. 1). However, the addition of acidic substrates (HA), which act as a Brønsted acid, to stable solutions of O2•− in aprotic solvents accelerates the disproportionation, depending on the protic strength (acidity) of HA. Carter et al. (Carter, M. T., Hussey, C. L., Strubinger, S. K. D., and Osteryoung, R. A. Inorg. Chem. 1991, 30, 1149) showed that superoxide ion could be generated by the reduction of dioxygen in imidizalium chloride-aluminum chloride molten salt. However, the resulting superoxide ion was unstable and thus cannot be used as a reagent in subsequent reactions. Buzzeo et al. (Buzzeo, Marisa C.; Klymenko, Oleksiy V.; Wadhawan, Jay D.; Hardacre, Christopher; Seddon, Kenneth R.; Compton, Richard G. J. Phys. Chem. A 2003, 107, 8872) studied the electrochemical reduction of oxygen in two different room-temperature ionic liquids, 1-ethyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)imide and hexyltriethylammonium bis((trifluoromethyl)sulfonyl)imide. They used chronoamperometric measurements to determine the diffusion coefficient and concentration of the electroactive oxygen dissolved in the ionic liquid by fitting experimental transients to the Aoki model. They also determined the diffusion coefficient of the electrogenerated superoxide species. Zhang et al. (Zhang, D.; Okajima, T.; Matsumoto, F.; and Ohsaka T. J. Electrochem. Soc. 2004, 151, D31) analyzed the electrode reaction of the molecular O2/O2•− couple at different electrodes in three 1-n-alkyl-3-methylimidazolium tetrafluoroborate ILs, 1-ethyl-3-methylimidazolium tetrafluoroborate, 1-n-propyl-3-methylimidazolium tetrafluoroborate, and 1-n-butyl-3-methylimidazolium tetrafluoroborate. The systems have been analyzed quantitatively using CV, normal pulse voltammetry, and hydrodynamic chronocoulometry. CV measurements showed that the redox reaction of the O2/O2•− couple in these ILs is a quasi-reversible process and that the resulting O2•− is stable. They evaluated the relevant thermodynamic and kinetic parameters of the O2/O2•− redox couple using cyclic and normal pulse voltammetry.
Shukla et al. (Shukla, A. K. and Singh, K. N. Indian Journal of Chemical Technology 2000, 7, 43) showed that Et4N+O2−, generated in situ by the phase-transfer reaction of KO2 with tetraethyl ammonium bromide readily oxidizes primary and secondary alcohols in dry DMF at room temp. As a result, primary alcohols are transformed into their corresponding acids, whereas secondary alcohols are converted to ketones in good yields. Kolarz et al. (Kolarz, B. N.; Rapak, A. Makromolekulare Chemie 1984, 185, 2511) studied the reaction of chloromethylated 1:99 divinylbenzene-styrene copolymer with KO2 in the presence of phase-transfer catalysts. In DMS and in the presence of 18-crown-6, the hydroxymethylated polymer was the main product with a yield of 45%. In DMF the transformation of chloromethyl groups was highest, but only 60% of alcoholic groups were present in the product. In benzene, the transformation was only 25%. With tetrabutyl ammonium iodide as catalyst in a mixture of solvents, the transformation of chloromethyl groups proceeded with 85% yield and the product contained 80% hydroxyl groups. Rao and Perlin showed that the reaction between glucitol and KO2 resulted in the loss of H-4 and the 5-mesyloxy (as well as 1-mesyl) substituent, and an almost quantitative conversion into enol ether. Tsuji and Takayanagi (Rao, V. S.; Perlin, A. S. Canadian Journal of Chemistry, 1981, 59, 333) showed that O—(HO)2C6H4 underwent oxidative ring cleavage on treatment with CuCl in pyridine containing ROH(R=Me, Et, Pr, Me2CH) to give 7-82% RO2CCH:CHCH:CHCO2H (I). The same oxidation also occurred using KO2/CuCl2 and KOH/CuCl2 in pyridine containing ROH in the absence of O. PhOH was also oxidized with the above systems to give I.
U.S. Pat. No. 6,225,273 disclosed photochemical superoxide generators useful as photobleaches for laundry detergent compositions and as photobleaches or photodisinfectants for use in hard surface cleaning compositions. The compounds described therein comprise an amino-containing electron transfer moiety bonded to the photosensitizing unit wherein the amino-containing moiety is capable of transferring an electron to the photochemically excited π electron cloud of the photosensitizer unit thereby enabling superoxide formation.
U.S. Pat. No. 5,358,657 disclosed reagent compositions suitable for use in degrading and detoxifying polyhalogenated organic compounds comprising an aprotic solvent having dissolved therein (a) an effective amount of hydrogen donor, (b) an effective amount of a compound which produces hydroxide ion or alkoxide ion, and (c) dioxygen. These reagent compositions may be used to produce superoxide ion in situ for use in a variety of industrial applications to degrade halogenated hydrocarbons, e.g., PCBs. The generation of superoxide ion may be catalyzed by the presence of anthraquinone and derivatives thereof. Reagent compositions containing (a) an effective amount of hydrogen donor, e.g., hydroxylamine, (b) an effective amount of a compound which produces hydroxide ion or alkoxide ion and (c) dioxygen are also shown. In preferred methods, the dioxygen is bubbled through the solutions to continuously form superoxide ion.
U.S. Pat. No. 5,143,710 provided methods for generating superoxide ions in an aprotic solvent. In each method a compound that is dependent on the particular reaction mechanism of the method reacts with dioxygen dissolved in the aprotic solvent and hydroxide ions or alkoxide ions in solution in the aprotic solvent to generate the superoxide ions. In the first method, hydrogen donor compounds such as aniline and N-substituted anilines, or phenylhydrazine and phenylhydrazine derivatives, react with the dioxygen and hydroxide ions or alkoxide ions to generate concentrations of superoxide ions in the aprotic solvent. In the second method, proton donor compounds such as hydroxylamine and N-substituted hydroxylamines react with the dioxygen and hydroxide ions or alkoxide ions to generate concentrations of superoxide ions in the aprotic solvent. In the third method, hydrazine reacts with the dioxygen and hydroxide ions or alkoxide ions to generate superoxide ions in the aprotic solvent when catalyzed by anthraquinone and anthraquinone derivatives. The solution of superoxide ions in an aprotic solvent may then be used to degrade halogenated hydrocarbons. In addition, several other methods have been developed to generate superoxide ions. For example, pulse radiolysis of dioxygen has been used to generate superoxide ions (Gebicki et al. J. Am. Chem. Soc. 1982, 104, 796). Further, photolysis of hydrogen peroxide in aqueous media, and base-induced decomposition of hydrogen peroxide have also been used to generate superoxide ions (McDowell et al. Inorg. Chem. 1983, 22, 847; and Morrison et al. Inorg. Chem. 1979, 18, 1971).
U.S. Pat. No. 4,199,419 disclosed a photochemical method and apparatus for generating superoxide radicals in an aqueous solution by means of a vacuum-ultraviolet lamp of simple design. The lamp is a microwave powered rare gas device that emits far ultraviolet-light. The lamp includes an inner loop of high purity quartz tubing through which flows an oxygen-saturated sodium formate solution. The inner loop is designed so that the solution is subjected to an intense flux of far-ultraviolet light. This causes the solution to photodecompose and form the product radical.
U.S. patent application No. 20060011465 disclosed a plasma driven, N-Type semiconductor, thermoelectric-power superoxide ion generator with critical bias conditions.
Solutions of superoxide ion in aprotic solvents have also been prepared using electrochemical means (Sawyer et al. Anal. Chem. 1982, 54, 1720). For example, the superoxide ions used for degrading halogenated hydrocarbons in U.S. Pat. Nos. 4,468,297 and 4,410,402, are generated in a controlled potential electrolysis cell which uses aprotic solvent for the electrolyte.
Potassium superoxide is a product particularly well suited for the regeneration of a breathable atmosphere because it has the characteristic of fixing carbon dioxide gas and water vapor and releasing oxygen according to the reactions:
                                          2            ⁢                          KO              2                                +                      CO            2                          ->                                            K              2                        ⁢                          CO              3                                +                                    3              2                        ⁢                          O              2                                                          (        3        )                                                      2            ⁢                          KO              2                                +                                    H              2                        ⁢            O                          ->                              2            ⁢            KOH                    +                                    3              2                        ⁢                          O              2                                                          (        4        )            
This characteristic is used to make atmosphere regenerators having closed chambers and respiratory apparatus that operate in a closed circuit.
U.S. Pat. No. 4,101,644 disclosed a method for the preparation of calcium superoxide in high yields from calcium peroxide diperoxyhydrate.
U.S. Pat. No. 4,088,595 disclosed an invention relates to an improved detergent composition that produces superoxide ion. This composition comprises at least one hydrosoluble salt of a metal selected from the group consisting of divalent iron, divalent cobalt and divalent nickel, associated with at least one hydrosoluble ligand which is a hydrogen donor and has at least two sites available for fixing to the said metal.
The methods described above for generating superoxide ions suffer from several disadvantages and are not appropriate for all applications. For example, methods for generating superoxide ions based on pulse radiolysis, photolysis, or electrolysis, all require radiation or electrical energy sources. Typically, the energy costs for these methods are prohibitively expensive, especially for applications such as degrading halogenated hydrocarbons on an industrial scale. Likewise, methods for generating superoxide ions based on decomposing hydrogen peroxide may be prohibitively expensive for particular applications due to the cost of hydrogen peroxide. Consequently, other methods for generating superoxide ions are desired.
A deep eutectic solvent (DES) is a type of ionic solvent with special properties composed of a mixture which forms a eutectic with a melting point much lower than either of the individual components. The first generation eutectic solvents were based on mixtures of quaternary ammonium salts with hydrogen donors such as amines and carboxylic acids. The deep eutectic phenomenon was first described in 2003 for a 1 to 2 by mole mixture of choline chloride (2-hydroxyethyl-trimethylammonium chloride) and urea. Choline chloride has a melting point of 302° C. and that of urea is 133° C. The eutectic mixture however melts as low as 12° C.
This DES is able to dissolve many metal salts like lithium chloride (solubility 2.5 mol/L) and copper(II) oxide (solubility 0.12 mol/L). In this capacity, these solvents could be applied in metal cleaning for electroplating. Because the solvent is conductive, it also has a potential application in electropolishing. Organic compounds such as benzoic acid (solubility 0.82 mol/L) also have great solubility and this even includes cellulose (filtration paper). Compared to ordinary solvents, eutectic solvents also have a very low VOC and are non-flammable. Other deep eutectic solvents of choline chloride are formed with malonic acid at 0° C., phenol at −40° C. and glycerol at −35° C.
Compared to ionic liquids that share many characteristics but are ionic compounds and not ionic mixtures, deep eutectic solvents are cheaper to make, much less toxic and sometimes biodegradable.
WO 2002 026381 disclosed an invention related to ionic compounds and methods for their preparation. In particular, the invention relates to ionic compounds comprising hydrated metal salts, which are liquid at low temperatures, generally below about 100° C.
WO 02/26701 A2 disclosed a method for the synthesis of DES compounds with a freezing point of up to 100° C. by the reaction of one amine salt (I), such as choline chloride with an organic compound (II) capable of forming a hydrogen bond with the anion of the amine salt, such as urea, wherein the molar ratio of I to II is from 1:1.5 to 1:2.5. The DES compounds are useful as solvents, and electrolytes for example in electroplating, electrowinning, electropolishing, and as catalysts.
WO 00/56700 disclosed a method for the synthesis of DES having a melting point of no more than 60° C., formed by the reaction of a quaternary ammonium compound or a mixture of two or more thereof; with a halide of zinc, tin or iron, or a mixture of two or more thereof.
We were the first to show that a stable superoxide ion can be generated in ILs [Al Nashef et al. Ph. D. Dissertation, 2004]. We also showed that hexachlorobenzene could be destroyed by the reaction of the superoxide ion generated in selected ionic liquids (“ILs”). However, the superoxide ion reacted with the cation of the IL wasting part of the solvent and producing undesired byproducts and hence, reducing the efficiency of the process.
From what was mentioned above it is clear that there is a need for a viable decontamination method that is inexpensive, occurs at ambient temperature, and most importantly, benign.